1921] on Elasticity 389 



chronograph. It is in these parts that constant friction is most 

 likely to occur. 



The slide (10) shows the general arrrangement. The style is 

 mounted on a horizontal reed so hung by silk fibres as to have only 

 one degree of freedom — namely, that of motion parallel to its 

 length, and at right angles to the travels of the chronograph paper. 

 (Fig. 5.) This kind of suspension I have used on a larger scale for 

 ballistic pendulums. 



1 chose a reed [Arundo phragmites) as a style-carrier on account 

 of its lightness and rigidity. Such reeds are practically wooden 

 tubes with walls less than a hundredth of an inch thick, coated with 

 a stiffening layer of silica. 



One end of the reed is terminated by a fine vertical steel point, 

 as also is the end of the horizontal arm on the oscillating rod. The 

 connecting rod which joins the two is also a reed, at either end of 

 which are mounted sapphire cups such as are used for magnetic 

 compasses. 



It was essential that the style should not touch the paper on 

 which it recorded its position, and this condition I at first intended 

 to satisfy by using a syphon-recorder of Sir W. Thomson's type. I 

 found, however, that it was much simpler to use an ordinary induc- 

 tion coil, the sparks from which, if properly adjusted, left an excellent 

 trace on the moving paper. The style, therefore, took the form of a 

 steel needle passed through the reed with its point about a hundredth 

 of an inch above the surface of the chronograph paper. 



One of the records obtained in this way is now shown as a lantern 

 slide. (See Fig. G.) 



This exhibits the rapid natural extinction of the oscillation in 

 the metal tin. 



In order to apply a known harmonic forcing couple to the oscil- 

 lating parts a horizontal arm was fixed to the upper end of the 

 suspension wire, and this, by a long connecting rod working from a 

 crank of adjustable throw, was made to twist the wire harmonically 

 through any desired angle. 



The crank was driven by a small electro-motor through a reduction 

 gear, and its speed regulated by the position given to a belt on two 

 cones, one cone being driven by the motor, and the other, by a second 

 belt, driving the crank shaft. 



The motor itself was controlled by a centrifugal governor, which 

 regulated the current supplied to it from the main. Each revolution 

 of the crank shaft was recorded on the chronograph, so that the 

 relative phases of the applied couple and the forced oscillation could 

 be compared. 



The experiments depending on oscillations have not yet been 

 completed, but the next three slides show some of the results obtained 

 by the experiments on the extinction of the natural oscillations. 



In slide (12) these are put in the same form as in (8), and they 



